Various drug-induced morphological changes in animals and man have been reported for a wide variety of agents. Descriptions of severe changes, more appropriately termed toxicities, have been quite valuable in clinical areas (for the prediction of dose-limiting toxicological problems) and in drug development areas (for evaluation of the relative efficacy of analogs). However, such purely descriptive approaches to drug-induced morphological changes are inadequate. They may not help predict undesirable side effects, and often they do not suggest how to more safely utilize a drug. Therefore, it has become necessary to study the mechanisms by which drugs induce morphological changes as means of better understanding their actions. Principal areas of effort are currently being directed toward understanding the following: 1) the mechanisms by with anthracyclines are cardiotoxic; 2) the mechanisms by which platinum compounds are toxic; 3) how chemotherapeutic agents and tumor promotors affect natural killer cells and othe other cell lines in tissue culture; and 4) why catecholo-methyltransferase is elevated in highly metastatic breast tumors.